Astronauts experience a 4.3-fold increase in disc herniation incidence following spaceflight as compared to the general population. Though space exploration enjoys excellence in aeronautical and mechanical engineering, healthcare promises have not been met despite current countermeasures and ongoing research on herniation mechanisms. The increased risk of herniation following spaceflight is thought to be due, in part, to prolonged exposure to microgravity which causes gains in stature of about twice that of diurnal values. While low back pain during spaceflight subsides soon after landing, inflated herniation risk persists for decades, and acute herniation can result in debilitating pain and ultimately may require surgery. The link between changes owing to spaceflight and subsequent spinal injury is confounded by inherent study limitations and lack of spaceflight data. Therefore the aim of this dissertation is to investigate the biomechanical etiology of herniation in astronauts since reduced gravitational loading alters musculoskeletal function and affects spinal stability and health.

Analytical risk models and biomechanical tests revealed that increases in disc height owing to simulated microgravity is a strong predictor of herniation risk and is correlated to increases in fiber strains and other biomechanical indictors of disc injury. In addition, a deeper understanding of the role of disc swelling and forward bending in disc prolapse is attained for the purpose of risk reduction. Namely, reducing disc height increases in space and restricting forward bending after return to Earth should be advised in earnest. Follow-on biochemical and morphological studies identify high inherent-risk demographics within the astronaut population for optimal crew selection in the interest of astronaut safety.

The experimental and theoretical work described herein provides useful benchmarks and guidelines for the design of longitudinal human studies. These and future studies can be used to develop countermeasures for mitigating deleterious microgravity-induced changes and optimizing post-flight reconditioning protocols against the adverse effects of prolonged spaceflight.